method refs: check if interface functional (IDEA-125511)

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/lambda/methodRef/Assignability.java

@@ -38,5 +38,5 @@ class Test2 {
 
     void foo(Integer i) {}
 
-    <error descr="Incompatible types. Found: '<method reference>', required: 'java.lang.Object'">Object o = Test2::foo;</error>
+    Object o = <error descr="Object is not a functional interface">Test2::foo</error>;
 }

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/lambda/methodRef/InvalidFunctionalTypeInReturnStmt.java

@@ -0,0 +1,8 @@
+class Java8 {
+  public void test() {
+  }
+
+  private int m() {
+    return <error descr="int is not a functional interface">Java8::test</error>;
+  }
+}